Physicists there led by Ka Chung Lee and Michael Sprague were able to show that the two 3-millimetre-wide diamonds shared one vibrational state between them.

Other researchers had previously shown quantum effects in a supercooled 0.06-millimetre-long strip of metal, which was set in a state where it was vibrating and not vibrating at the same time. But quantum effects are fragile. The more atoms an object contains, the more they jostle each other about, destroying the delicate links of entanglement.

Quantum entanglement in the motion of macroscopic solid bodies has implications both for quantum technologies and foundational studies of the boundary between the quantum and classical worlds. Entanglement is usually fragile in room-temperature solids, owing to strong interactions both internally and with the noisy environment. We generated motional entanglement between vibrational states of two spatially separated, millimeter-sized diamonds at room temperature. By measuring strong nonclassical correlations between Raman-scattered photons, we showed that the quantum state of the diamonds has positive concurrence with 98% probability. Our results show that entanglement can persist in the classical context of moving macroscopic solids in ambient conditions.

The team placed two diamonds in front of an ultrafast laser, which zapped them with a pulse of light that lasted 100 femtoseconds (or 10^-13 seconds).

Every so often, according to the classical physics that describes large objects, one of those photons should set the atoms in one of the diamonds vibrating. That vibration saps some energy from the photon. The less energetic photon would then move on to a detector, and each diamond would be left either vibrating or not vibrating.

But if the diamonds behaved as quantum mechanical objects, they would share one vibrational mode between them. It would be as if both diamonds were both vibrating and not vibrating at the same time. "Quantum mechanics says it's not either/or, it's both/and," Walmsley says. "It's that both/and we've been trying to prove."

To show that the diamonds were truly entangled, the researchers hit them with a second laser pulse just 350 femtoseconds after the first. The second pulse picked up the energy the first pulse left behind, and reached the detector as an extra-energetic photon.

If the system were classical, the second photon should pick up extra energy only half the time – only if it happened to hit the diamond where the energy was deposited in the first place. But in 200 trillion trials, the team found that the second photon picked up extra energy every time. That means the energy was not localised in one diamond or the other, but that they shared the same vibrational state.

Entangled diamonds could some day find uses in quantum computers, which could use entanglement to carry out many calculations at once.

"To actually realise such a device is still a way off in the future, but conceptually that's feasible," Walmsley says. He notes that the diamonds were entangled for only 7000 femtoseconds, which is not long enough for practical applications.